When a body of aluminum is melted in a furnace for purposes of casting or the like, dross forms on the surface of the molten aluminum which must be periodically removed, for example by skimming or similar operation. The removed dross contains substantial amounts of free aluminum as well as aluminum oxides, such as bauxite, and certain other metals and metal salts, such as magnesium, manganese and lithium, depending on the nature of the aluminum or aluminum alloy being treated. The dross may also include some nitrides and chlorides, possibly due to the manner in which the dross is treated.
It is recognized in the industry that for economical reasons it is critical to recover in usable form the free aluminum, aluminum oxide, and other by-product metals from the dross. It is also recognized, however, that the recovery of these materials from dross is difficult due, inter alia, to the nature of the dross and the reactivity of aluminum. In a typical recovery process the dross is normally melted at high temperatures in a furnace. However, at elevated temperatures the dross, particularly the free aluminum in the dross, is easily susceptible to oxidation and, moreover, commonly tends to ignite and burn in the presence of air. The burning of the aluminum can decrease substantially the amount of aluminum recovered.
To solve the problems associated with treating dross and improve the efficiency of aluminum recovery, it has been proposed to heat the dross in an induction furnace in the presence of a salt flux. See, for example, McLeod et al, U.S. Pat. No. 3,676,105. The use of a salt flux, which tends to agglomerate the free aluminum, is not desirable because of high costs and in that the salt, which tends to be water-leachable, in turn, must be separated from the aluminum, leading to cost and environmental problems.
It has been suggested in the prior art to recover aluminum metal in the gaseous state by breaking down aluminum salts at temperatures of at least 2500.degree. to 5500.degree. C. The heating to the essential high temperature, as disclosed in Othmer, U.S. Pat. No. 3,938,988, can be carried out using plasma energy from a plasma arc torch. According to the disclosed process, a liquid coolant is utilized to flash-cool a mixture of aluminum and other elemental gases to effect separation. This procedure is not conducive to the treatment of dross, and further is not practical for dross treatment either from the standpoint of cost or the ecology.
The use of a plasma jet generator has also been suggested for reducing various metal oxides, including aluminum oxides. See, for example, Tylko, U.S. Pat. No. 4,177,060, where carbon and alumina are supplied to a molten reactor bed maintained in a reaction chamber with the carbothermal reduction occurring through application of plasma energy from a plasma torch. The use of carbon in the reduction of alumina, i.e., a chemical reaction, is substantially different from the treatment of dross where separation of materials only is desired.
Accordingly, there is a substantial need in the industry for an improved process for recovering free aluminum, aluminum compounds such as the aluminum oxides, and other by-product metals from aluminum dross. It is particularly desirable that such process be cost effective and ecologically safe.